Combining geometrical-optical lenses in any way does not allow condensing a light into an area as extremely small as one half of its wavelength because of a diffraction limit of the light. It is then not possible either to pick up selectively a light emitted from an area as extremely small as one half of its wavelength. However, light has not only waves to propagate as described by the Maxwell's equations but also a near field that exists only extremely near the surface of an object. If a probe having a sharply pointed tip whose diameter is as small as less than the diffraction limit is used to bring this pointed tip into extremely close proximity to the surface of an object, there can then be created a near-field coupling between them to bring about a near field on the object surface, also permitting such a near field from an object to be taken as a light propagating. Using this method allows an optical image of an object to be observed at a resolution less than the diffraction limit and further the nature of matter such as Raman scattering to be measured at a resolution less than the diffraction limit.
In the past, however, there has been no technique available to control positioning a probe having a tip as sharp as less than the diffraction limit and to control the relationship in position between the probe tip and a measured object at a resolution less than the diffraction limit, and for this reason the near-field could hardly be applied.
In recent years, as represented by the atomic force microscope (AFM) it has become possible to control positioning a probe whose diameter is on an atomic size level and to control the relationship in position between the tip of the probe and an object to be measured three-dimensionally at a resolution less than an interatomic distance, thus spreading the use of the near-field applied techniques.
In the near-field applied techniques, however, that utilize light into and out of an extremely small area and therefore entail a signal that is extremely small in intensity, obtaining any information that is meaningful requires that light incident be condensed into a microfine area very efficiently and that a near field in such a microfine area be collected very efficiently, and namely calls for a system that is extremely high in light condensing efficiency. Further, obtaining a near-field image for a specimen requires that any light condensing positions be scanned at a resolution less than the diffraction limit.
The conventional near-field application techniques have the problem that they can scan light condensing positions but are low in light condensing efficiency or that they are high in light condensing efficiency but cannot scan light condensing positions, there being no apparatus which can be put to practical use by being excellent in light condensing efficiency and at the same time capable of scanning at a sufficiently high position resolution. Mention is made below of further details of the difficulties met by the prior art.
One of the near-field application techniques is a scattering-type scanning near-field optical microscope abbreviated as s-SNOM. This is an apparatus that is designed to cause a light of long wavelength such as a microwave to be condensed into a microfine area of sub-microns or less in size and an infrared light or microwave emitted from a microfine area of sub-microns or less in size to be collected or condensed. It utilizes a near-field coupling brought about between the sharply pointed tip of a small electric conductor and an object to be measured to perform a variety of measurements at a resolution less than the optical diffraction limit.
FIG. 8 illustrates the operating principles of such a microscope and shows the s-SNOM in a conceptual view as it condenses an infrared light at an AFM cantilever coated with an electric conductor. See literature: B. Knoll and F. Keilmann, Nature 399 (1999), issued 13 May 1999.
An infrared light 701 is incident from the external free space on an electric conductor 703 coated on a cantilever 702, then concentrating as a near-field at a tip 704 of the electric conductor 703. This near-field is coupled with a region, which is adjacent to the tip 704, on a specimen 706 mounted on a specimen support table 705. And inversely the near-field of the specimen 706 is coupled with and thereby transmitted to the tip 704 of the electric conductor 703 which then acting as an antenna emits it as an infrared light 707 into the external free space. Here, both the incident and outgoing infrared lights 701 and 707 are condensed by usual optical lenses (not shown). According to this apparatus, moving the cantilever 702 to let it scan allows an image by the entire near-field for the small specimen 706 to be observed at a resolution less than the diffraction limit.
However, since the proportion at which the infrared light 701 is coupled to the antenna 703 is proportional to factor (∈o/∈s) where ∈o and ∈s are the dielectric constants of the external space and the specimen support substrate 705, respectively, and ∈o<∈s, the efficiency of the electric conductor 703 as an antenna is rather low. Likewise, the near-field from the small specimen 706 largely is coupled to the specimen support substrate 705 high in dielectric constant and comes to be emitted in its large part into the interior of the specimen support substrate 705 with only its reduced part emitted in the form of the infrared light 707. Thus, this system has the problem that the efficiency at which to condense an incident infrared light and the efficiency at which to condense a near-field from a specimen as an outgoing infrared light, namely its light condensing efficiency is low.
Another near-field application technique in the prior art is next shown.
FIG. 9 illustrates a method wherein light is made incident from the side of a specimen support substrate to obtain Raman scattering light. See literature: H. Hayazawa et al., Chem. Phys. Lett. 355 (2001) 369, issued 2 May 2001. An incident light 801 is condensed by a usual optical lens 802 and then past a substrate 805 coated with oil 803 and a thin film of silver 804 is focused on a specimen 806 whereupon a near-field by the focused light is intensified by surface plasmon polariton brought about and excited at a tip 809 of a cantilever 808 coated with an electric conductor 807. And similarly, Raman scattering light 810 from the specimen 806 intensified by the surface plasmon polariton brought about and excited at a tip 809 is taken out and then past the substrate 805 and the oil 803 is condensed by the lens 802. According to this method, scanning the cantilever 808 allows observing a Raman scattering scanning image of the specimen 806 at a resolution less than the diffraction limit.
In this method, however, the geometrical size of an electric conductor 807 as a whole is not taken into consideration, thereby leaving the electric conductor 807 less functioning as an antenna. Moreover, its function as an antenna is restricted by a screening or shielding effect exerted by the silver coating 804 on the specimen support substrate 805. Thus, lacking an adequate light focusing capability, the system is low in the efficiency at which to convert the near-field from the small specimen 806 into the outgoing light 810. To wit, the system has the problem that its light condensing efficiency is low.
By the way, in the field of far-infrared techniques there is known an efficient light condensing method for a microfine absorber in a far-infrared region. FIG. 10 is a conceptual view illustrating such an efficient light condensing method conventionally known for a microfine absorber in a far-infrared region. See literature: “Infrared and Millimeter Waves, Volume 10”, Millimeter Components and Techniques, Part II, Chapter 1 (1983), ed. by Academic Press Inc. An incoming light 901 is incident into a solid immersion lens made of dielectric 902 and condensed on a planar dipole antenna or planar slot antenna 903 lying at its focal position. Further, the incident light condensed on the antenna is caused to geometrically resonate by the antenna and focused onto a small far-infrared absorber 904 disposed at the center of the antenna. According to this method in which a far-infrared light is caused to geometrically resonate by an antenna, it is possible to absorb the far-infrared light efficiently and at due sensitivity. Consequently, if a specimen to be measured is disposed in place of the far-infrared absorber 904 at a position at which it is disposed, then it is possible to take out the near-field 905 efficiently.
In this method, however, in which the planar dipole antenna or planar slot antenna 903 is fixed onto the solid immersion lens 902, it is not possible to move the light focusing position for scanning.
Thus, although there have already been systems designed to condense a light at a microfine area smaller than its diffraction limit or to take up a near-field from a microfine area smaller than its diffraction limit and then to condense it, e.g., to condense an infrared light whose wavelength is several tens microns or more at an area as microfine as sub-microns or less or to take out a near-field from an area as microfine as sub-microns or less and then to condense it, these conventional systems have the problem that they are low in light condensing efficiency or otherwise are incapable of scanning such a microfine position of light condensing, or light taking up.